FIELD OF THE INVENTION
This invention pertains to microwave treatment of dielectric materials.
Microwave treatments which are traditionally used for developing or employing dielectric materials, especially organic matrix composite materials, whose dimensions are greater than the half-wavelength of electromagnetic radiation will produce either a system which ensures relative displacement of the material with respect to the spatial structure of the electromagnetic field, or a multi-mode cavity type system with propagation mode mixer.
In the second solution, which employs the application of a pressure constraint, the material to be treated and the dielectric components which make up the group which keeps this material under pressure (die rather than the punch) by their average permittivity, their dimensions and the configuration of their interfaces, constitute a microwave electric circuit which selects the propagation modes. Indeed, experiments show that, with or without a mode mixing unit, homogeneity defects or "heterogeneous markings" will appear in the body of the material to be treated if the treatments are not sufficiently mild and their durations sufficiently prolonged.
The homogeneity defects or markings observed in these experiments result from the establishment of standing wave conditions which are generated in the material due to internal reflections to the interfaces, as a function of the propagation modes excited by these interfaces and as a function of the particular distances which separate them. These effects are that much more considerable as the attenuation of the waves in these types of materials is relatively weak because of the low dielectric constant of losses and as the diffusion of heat does not promote homogenization between the marked zones, because of the thermal conductivity which is also weak and because of the duration of the voluntarily reduced treatments.
This observation of markings caused by the heterogeneities and the incidence of diffusion conditions of heat are clearly specified in the patent FR.84 15 106. In the device described in this patent, the homogenization results exclusively from conditions of thermal conductivity, the applicator remaining the multi-mode type.
For the device described in patent JP-60-135230, the pressure constraint is reduced by placing the materials to be treated or assembled by gluing under vacuum. For this reason the filling of the applicator remains low and does not require specific dielectric conditions for the dielectric constant values.
The patent FR 88 08 081 does not specify any component pertinent to the application of microwave energy or to the conditions of wave propagation.
In order to overcome the disadvantages of the prior art, a "mold" has been proposed in which the electric field of the fundamental wave is guided so as to avoid excitation of hybrid modes which disturb homogeneity of the treatment process.
The great difficulty encountered in the processes for treating materials by microwaves is controlling the spatial distribution of electric energy, particularly the obtaining of homogeneous treatment.
A device for treating dielectric materials by using microwaves which includes a mold intended to contain the material to be treated, means for placing the material contained in the mold under pressure, and a microwave generator used to apply the microwave energy to the mold which is required for treatment of the material has been described in the patent FR 90 14889. The mold includes several dielectric materials which have different dielectric permittivities and which are arranged in the mold so that the dielectric permittivity of the set will vary by decreasing from the material to be treated to the external wall of the mold, the value of the average dielectric permittivity of the dielectric materials being less than the value corresponding to the situation of cut-off of the first order mode greater than the fundamental mode of the electromagnetic wave produced by the generator.
The microwave device described in this patent has the goal of achieving spatially controlled transformation of a material.
This homogeneity is obtained by using a dielectric "mold" in which the fundamental wave of propagation is maintained in order to achieve the largest possible volumes in which the sources of heat are uniformly distributed in space. This patent also defines the conditions for distribution in the space of elements which constitute the dielectric mode in order to obtain a large concentration of the electromagnetic power in the object to be produced. A central feature of this patent is the description of the adaptations of the two interfaces of the dielectric mold, the input interface and output interface for propagation of the mode, so as to maintain a progressive wave without standing wave conditions which will result from possible reflections by poorly adjusted interfaces.
This invention employs three components which are new and different from those described in the aforementioned patent for the design of dielectric devices which constitute the mold with respect to arrangement, shape and dielectric characteristics. These components complement or moderate the recommendation to arrange the dielectrics of decreasing permittivity from the median plane of the applicator toward its external walls.
The invention thus has as an object a device for microwave treatment of materials other than massive metal materials which include a mold whose purpose is to hold the material to be treated, a metal applicator and at least one microwave generator used to apply to the mold the microwave energy which is necessary for treatment of the material, the mold including at least two dielectric materials surrounding the material to be treated, characterized in that the dielectric materials have dielectric permittivities which are different from one another so that the attenuation of an elementary segment of length l located between the points of side z(n-1) and zn of the longitudinal dimension of the material to be treated will include at least one section broken up into segments of length l, is controlled by the relationship: ##EQU2## in which .alpha. is the attenuation in the elementary segment in question,
.alpha.o is the initial attenuation in the body of the section which includes the elementary segment in question, Po is the microwave power applied to the section, P(n-1) is the microwave power transmitted in the plane of side z(n-1).
According to a particular characteristic of the invention, in the case of a section which is broken up into segments of the same length l, the attenuation of the n.sup.th segment is controlled by the relationship: ##EQU3##
According to a particular characteristic of the invention, the metal applicator contains the dielectric materials and the material to be treated and its straight section, along its longitudinal dimension, follows the possible curves of the item to be developed from the material to be treated.
According to another characteristic of the invention, at least one of the two dimensions of the transverse section of the applicator is variable in the direction of propagation of microwaves and allows compensation for attenuation of the wave resulting from too great a focusing of waves in the central zone of the applicator.
According to yet another characteristic of the invention the dielectric constants of losses of dielectric materials are different.